The present invention relates generally to the production of curved, tempered sheets of glass and, more particularly, to an improved method of and apparatus for bending and heat treating relatively thin glass sheets.
Bent sheets of glass are commonly used as glazing closures in vehicles such as automobiles and the like. For such applications, the glass sheets must be bent to precisely defined curvatures dictated by the configurations and size of the openings and the overall styling of the vehicle. Additionally, it is necessary that bent glass sheets utilized as glazing closures in vehicles be tempered to increase their resistance to damage resulting from impact and, in the event of breakage, to fragment into relatively small harmless particles as opposed to the large, jagged, potentially dangerous pieces otherwise resulting from untempered glass sheets when broken. Further, it is important that the bent and tempered glass sheets meet stringent optical requirements and that the viewing area of glazing closures be free of surface defects and optical distortions that would interfere with clear vision therethrough.
Generally, the commercial production of curved, tempered sheets of glass for such purposes includes heating pretrimmed, flat sheets of glass to the softening temperatures thereof, bending the heated sheets to a desired curvature between a pair of complementary mold sections and then chilling the bent sheets in a controlled manner to a temperature below the annealing range of glass. In a mass production operation, the above operations are carried out successively while the sheets of glass are being advanced substantially continuously along a fixed path to a heating area, a bending area, and a chilling or tempering area. To achieve satisfactory temper, the temperature of the glass sheet must be above a predetermined minimum level so as to maintain the core or central portion thereof above a deformation temperature upon being exposed to the tempering medium. The residual heat remaining in glass sheets of conventional thicknesses, such as those having thicknesses ranging from 0.200 to 0.255 inch, for example, is generally above such predetermined minimum level after bending for immediate advancement to the tempering area and exposure to the tempering medium. Thus, the heat initially imparted to the sheet to bring it to the proper bending temperature can also be utilized in the final heat treating tempering operation.
In recent years, however, considerable emphasis has been placed on the use of thinner glass sheets for automotive glazing purposes, the thicknesses thereof preferably ranging from about 0.100 to 0.156 inch for example. While the process described above is admirably suited for the mass production of the thicker glass sheets, it does not lend itself to the processing of relatively thinner glass sheets because of the lesser ability of such thinner sheets to retain heat. As the thickness of the glass decreases, the rate of heat loss increases. Thus, the heat loss occurring between initial heating and tempering occasioned by the intermediate bending operation in accordance with the above technique brings the temperature of a thin glass sheet down to a level below the aforementioned minimum temperature at which satisfactory tempering can be effected. On the other hand, overheating the thin sheets of glass during initial heating to compensate for the rapid subsequent loss of heat during bending renders the sheets extremely pliable with attendant loss of the deformation control necessary to maintain the shape of the bent sheets within the close tolerances dictated by automobile design and styling requirements. Moreover, such overheating tends to degrade the surface quality of the finished glass as a result of heat stains, roll deformation, pitting and the like. While attempts have been made to solve these problems in the mass production of thin, bent, tempered glass sheets, none have been completely satisfactory in obtaining a quality temper while maintaining the desired shape imparted to such thin glass sheets during bending.